It is known that supported cobalt-containing Fischer-Tropsch synthesis (FTS) catalysts can be prepared by means of impregnation of a cobalt salt onto a catalyst support coupled with drying of the impregnated support, followed by calcination of the resultant dried impregnated support, to obtain a FTS catalyst precursor. The catalyst precursor is then reduced to obtain the FTS catalyst comprising cobalt crystallites dispersed on the support.
When a cobalt compound such as cobalt nitrate is used to impregnate the catalyst support, only a relatively low cobalt loading on the catalyst support can be obtained using a single impregnation step. Accordingly, it is known to make use of multiple cobalt compound impregnation steps in order to increase the cobalt loading on the support. U.S. Pat. No. 6,455,462, U.S. Pat. No. 5,733,839 and WO 2010/075516 describe multiple impregnation methods for preparing a FTS catalyst. These methods include a first stage wherein a cobalt salt is introduced onto a catalyst support by impregnation, followed by calcination of the impregnated support. In a second stage the calcined impregnated support of the first stage is subjected to a second cobalt salt impregnation, followed by calcination. In a third stage, the calcined support of the second stage is then activated by subjecting it to a reducing gas to provide a FTS catalyst.
It is also known that in preparing a FTS catalyst the calcination step can be omitted, that is, after the catalyst support is impregnated with the cobalt salt, said impregnated support is directly reduced (without first being calcined) to provide the FTS catalyst. Such a direct reduction process is described in WO 2008/090150 and Journal of Catalysis 153 (1995) 108-122 for reducing a catalyst support which has been subjected to a single impregnation with molten cobalt nitrate.
Surprisingly, it has now been found that when a FTS catalyst is prepared according to the process of the present invention, such a catalyst demonstrates improved activity and/or lower methane selectivity over a catalyst which has been prepared according to the double impregnation procedure described above, that is where each impregnation step is followed by a calcination step, whereafter the calcined product is reduced. Surprisingly, it has also been found that the process according to the present invention provides a FTS catalyst with similar FTS activity and lower methane selectivity compared to a catalyst prepared by a double impregnation procedure wherein each impregnation step is followed by direct reduction of the impregnated catalyst support. Last-mentioned process also has the disadvantage that the product formed after the first impregnation and reduction cycle is a pyrophoric product which needs to be subjected to passivation before the second impregnation step can be carried out. This disadvantage is avoided by the present invention.